The filamentous fungi, owing to their ability to secrete high levels of proteins, are attractive organisms for the expression and secretion of heterologous proteins of commercial and medical value. Secretion is a key element in their attraction, for secretion provides the opportunity for complex proteins to fold and be processed properly and secretion will facilitate purification. Proper folding and processing, simplified purification, and inexpensive culturing translate into less expensive production. We have demonstrated secretion and processing of an enzymatically active mammalian protein (preprochymosin) from Neurospora crassa, and we propose to test the feasibility of expressing a single chain antibody fragment in Neurospora. The use of Neurospora to express single chain antibodies in novel.